1. Field of Invention
This invention relates to a novel high-strength composite material, specifically a composite material comprising a wound multilaminate structure of amorphous metal ribbon and ceramic laminae, and to a method for producing same.
2. Background of the Invention
One of the continuing goals of the specialty materials industry is the production of materials with enhanced properties such as corrosion resistance, hardness, and tensile strength. One well known method of achieving this goal has been the creation of new forms of old materials by alloying or by changing the microstructure of the material to one with different properties. Another method is the formation of composite structures which physically integrate one material with another (often the two are dissimilar--such as epoxy resin and carbon fibers) in order to create a new material which has greater strength (or other desirable enhanced property) than any constituent thereof alone.
Under normal solidity conditions metals have an ordered atomic structure (microstructure). This structure comprises a lattice of metal atoms forming a regular repeating geometric atomic array. As a metal solidifies, or if it is subject to a sufficiently high temperature to initiate recrystallization, the atoms are only able to arrange themselves into local regions of ordered structure. The size of these individual regions, or grains, is determined by the speed at which solidification takes place (larger grain sizes requiting more time, entailing slower cooling, to order themselves).
Crack propagation is the primary mechanism by which materials under stress fail. Cracks within ordered lattice regions (grains) are inherent in all crystalline structures in statistically quantifiable concentrations due to ordering faults controlled by thermodynamic principles. Grain boundaries, however, provide the majority of crack initiating stress concentration points due to voids present at boundaries between multiple grains which are not aligned ideally with respect to one another. Grain boundaries also provide nucleation sites for travelling ordering faults within the grains.
Crack propagation through crystalline metals is further enhanced by slip systems and tip stress concentrations which are inherent in the crystal lattices of these metals, both in the grains and between the grains. The simple theoretical atom to atom bond strength of most metals predicts an overall material strength that is on the order of 10 to 1,000 times the strength that is actually measured in crystalline metals.
Amorphous metals are metals which have been formed in such a way so as to have no crystalline structure, i.e., are non-crystalline metals. The formation of amorphous metals is achieved by cooling a metal from a molten state to the solid phase at sufficiently a high rate so that the grain size shrinks to the size of a single atom. Therefore, because a single atom is its own grain, this metal has no long, or even short, range order. No grains or grain boundaries means that the resulting amorphous metal has dramatically fewer voids and crack nucleation sites.
The absence of crystalline regions means that the metal is substantially devoid of slip systems and substantially free of significant mobility of faults to nucleation sites. The inherent strength of the metal is dramatically increased since any residual cracks in the metal have no lattice-type regular paths along which to propagate. In theory, a pure amorphous metal would exhibit remarkably enhanced tensile strength relative to corresponding crystalline metals.
The existence of the amorphous state of a metal is readily empirically verified by X-ray crystallography techniques of a conventional type, as revealing the non-crystalline character of the amorphous metal.
Traditionally, however, amorphous metal compositions have necessarily included crystallization inhibitor components (hereinafter referred to as inhibitors) because even the fastest molten metal cooling rates achievable to date have been slower than the crystallization rates in pure (inhibitor-free) metals. Unfortunately, these inhibitors have a secondary effect of reducing the tensile strength of the resulting metal. In addition, the cooling techniques that have been used to achieve significantly reduced recrystallization rates, concurrently limit the dimensions of the amorphous metal, so that the amorphous metal product article is typically of very small size.
Commercially produced amorphous metals are formed by a process of pouring a molten metal-inhibitor mixture onto a rapidly spinning wheel which is cooled with liquid nitrogen. The amorphous metals produced by this rapid "spin quenching" technique are thus formed in thin ribbons. The thickness of the metal ribbons, along with the structural weakening of the product material resulting from the presence of the inhibitor component(s), has hindered previous attempts to take advantage of the potential structural value of amorphous metals.
Although recent advances have reduced the amounts of inhibitors necessary in the molten metal to suppress crystallization, and by such reduced inhibitor concentration dramatically increased the attainable tensile strengths (reported amorphous metal ribbon tensile strengths for such "low inhibitor" molten metal compositions have been measured in excess of 400,000 psi), to strength levels generally associated with structural applications, the physical constraint of the ribbon or strip form of the amorphous metal product that is imposed by the spin quenching technique has posed a continuing barrier to efforts to use amorphous metals in such structural applications.
Ceramic materials, including ceramics of covalent as well as ionically bonded character, are used extensively throughout industry, in many applications. While ceramics have inherent tensile and compressive strengths far exceeding the strength values of many other materials, ceramics are highly susceptible to defects incorporated during their formation process. These defects lead to strength faults when the ceramic is produced in bulk form, and such strength faults in turn reduce the structural integrity of the product ceramic material.
Fiber-form ceramics, often referred to as ceramic whiskers, have a reduced defect size and concentration, and as a result exhibit strengths ranging up to and beyond 700,000 psi. Fibers, however, have limited structural utility by themselves, and typically are employed as discontinuous reinforcement elements in composite materials, as hereinafter more fully described. Thus the use of ceramics in structural applications, where their intrinsic high strengths would otherwise be highly advantageous, has been limited.
The combination of metals and ceramics, wherein each's strengths used are to compensate for the weaknesses of the other, is well known in the art.
The most widely known form of such combination has been ceramic fiber-reinforced metals which are comprised of a metal matrix, loaded with solid ceramic fibers, produced by introduction of ceramic fibers into the metal while in the molten state, and before solidification thereof. The resulting molten metal/ceramic fibers mixture is subsequently cooled in such a way that the fibers are spread through the matrix in a predetermined distribution (most often such distribution is a constant concentration, random orientation distribution throughout the resulting composite).
The ceramic fibers in such resulting composite retain their inherent high strength and contribute crack resistance which strengthens the matrix metal by preventing cracks from propagating through the metal. The ceramic fibers in the metal matrix composite also constrain the metal from flowing elastically or plastically under applied loads, so that the metal matrix composite material exhibits increased hardness and creep resistance as well as increased strength. The actual strength of the product metal matrix/ceramic fiber composite material is a function of the strength of the metal matrix, the bonding strength between the fiber and the metal, and the strength of the fibers.
Since the primary determinant of the strength level of the metal matrix/ceramic fiber composite is the metal component's physical properties, the strength of the ceramic-reinforced metal composite is ultimately limited by the microstructure of the metal. While currently available metal matrix/ceramic fiber composites achieve definite improvements in a number of important structural properties, relative to either ceramics or metals alone, these metal matrix/ceramic fiber composites nonetheless do not even begin to approach the maximum physical and performance properties which arguably should result from the combination of ceramics and metals based on theoretical considerations.
Another metal and ceramic combination which has been proposed by the art is a metal-reinforced ceramic composite. This type of composite, which is comprised primarily of ceramic (with ceramic as the continuous phase, and metal as the discontinuous or reinforcement phase), differs from conventional ceramic materials in that the metal-reinforced ceramics are much less brittle and less prone to formation defects than the corresponding pure ceramic component.
Various types of such metal-reinforced ceramic composites have been proposed in the art.
One type of such metal-reinforced ceramic composites comprises composites which physically incorporate crystalline metal into the ceramic matrix. These composites, well known in the art, are often formed by mixing the ceramic and metal components, or by depositing one component into a matrix of another, and then, if necessary, heating the both until the metal and ceramic bond. An illustrative deposition method of such type is described by Mohammad Ghouse in "Influence of Heat Treatment on the Bond Strength of Codeposited Ni--SiC Composite Coatings," Surface Technology, 21 (1984), 193-200. In the methodology disclosed in this article, a heat treatment is employed to bond the composite metal to a ceramic substrate, after codeposition of the composite metal and ceramic components.
Other chemical or electrochemical methods for forming the aforementioned type of such metal-reinforced ceramic composite are known in the art, as well as thermal methods of hot pressing.
Due to the substantial temperature sensitivity of amorphous metals, and the overall difficulty associated with the formation of the amorphous phase, metal reinforced ceramics produced through hot pressing or sintering techniques cannot accommodate the desirable amorphous form of metal in the fabrication of the composite.
Only recently has the use of amorphous metal with ceramics in composites applications been proposed by the art. U.S. Pat. 4,770,701 to Henderson et al. discloses a form of and method for producing such composites comprising amorphous metals and ceramics.
The Henderson et al. '701 patent discloses a method of forming an amorphous or microcrystalline metal-ceramic composite, comprising intimate physical mixing of alloying metal and ceramic particles in the presence of heat and reaction catalyst. The resulting composition undergoes a solid state formation reaction, which chemically creates a dual-phase microstructure composed of ceramic particles held together by amorphous and/or microcrystalline metal alloy. The metal is in the amorphous and/or microcrystalline state because it is chemically deposited in solid form in thin layers between the ceramic particles.
An inherent weakness of the material formed by process of the Henderson et al. patent, and the forming process itself, is the dependence of such composite system on a solid state reaction which is difficult to control in such manner as to ensure uniform thickness of the metal. The metal thickness is an extremely important aspect of the binding metal that ultimately determines the strength of the material. A related weakness of the amorphous metal--ceramic particle bonded material of this patent is that it is limited to less than 25% amorphous metal. Any higher concentrations of amorphous metal give rise to thicker layer clusters of metal than is acceptable, in turn causing the crystal grains in the metal to be larger than desired, and thereby losing the significant structural and performance benefits derived from the presence of microcrystallinity and amorphousness of the metal component in the composite.
An inherent weakness of the above-described metal/ceramic composites is their dependence on the bonding of metal to ceramic which is the basis for amorphous metal--ceramic particle bonded amorphous metal-ceramic composites.
Another weakness of such prior art composite material formation techniques and deficiency of the material itself is that the rate at which the material may be formed is dependent on the solid state reaction time.
Related to these weaknesses is the expense and complexity associated with incorporating metal or ceramic constituents into the composite structure.
Accordingly, it is an object of the present invention to provide a new composite having high structural strength comprising amorphous metal and ceramic, that is not dependent on solid state reaction rates.
It is a further object of the present invention to provide a composite material comprising amorphous metal and ceramic wherein the percentage concentration of amorphous metal is not limitingly constrained by solid state reactions, and wherein the amorphous metal content may range from as low as 1 percent to as high as 99 percent.
It is a further object of the present invention to provide a composite material comprising amorphous metal and ceramic which is substantially independent of the bonding of the metal and ceramic components.
It is a further object of the present invention to provide a generalized composite material comprising amorphous metal and ceramic for which alternate ceramic or metal constituent elements are more easily substituted into the material formation process.
It is further object of the present invention to provide a novel less expensive process for making a structural material comprising amorphous metal and ceramic.
It is further object of the present invention to provide a process for making a composite material comprising amorphous metal and ceramic which is substantially faster and less complex than prior art techniques.
Other objects and advantages will be more fully apparent from the ensuing disclosure and appended claims.